Image forming process by means of an electro-photographic image forming device is made up of a series of processes such as charging, exposure, development, image transfer, cleaning, fixing and static elimination. An example of arrangement of the image forming device is shown in FIG. 6. As shown in the FIG. 6, a photoreceptor drum 3 is rotatably provided in a direction of arrow Si. The photoreceptor drum 3 is made up of a conductive body 3a made of metal or resin, and a photoconductor 3b having at least an under coating layer and a photosensitive layer being stacked on the body 3a in this order. The photosensitive layer, in particular, is made up of a relatively thin Carrier Generation Layer (hereinafter referred to as CGL) which is formed on the under coating layer, and a relatively thin Carrier Transport Layer (hereinafter referred to as CTL), which is the outermost layer made of polycarbonate as a main component.
A main charging device 21, which is made up of a power source 22 and a charger 23 such as a corona charger or a contact-type charging roller, supplies charge on a surface of the photoconductor 3b to a predetermined potential. Next, when an exposure device 31 exposes a predetermined portion on the surface of the photoconductor 3b, among charges (carriers) generated from the CGL, charges having the opposite polarity to the charge on the surface of the photoconductor 3b are moved to the surface of the photoconductor 3b through the CTL. This cancels the charge on the surface of the photoconductor 3b in the exposed portion and forms an electrostatic latent image potential, thus carrying an electrostatic latent image on the photoconductor 3b.
Next, by a rotation of the photoreceptor drum 3, the photoconductor 3b carrying the electrostatic latent image moves to a development region D where the photoconductor 3b contacts a developing device 41. In the development region D, a developer carrier 42, which rotates in a direction of arrow S2 opposite to the direction of arrow S1, and to which a predetermined bias voltage from a power source (not shown) is applied, is pressed against the surface of the photoconductor 3b. Then, toner carried by the developer carrier 42 is transferred from a developer tank 43, and adheres to the electrostatic latent image on the photoconductor 3b, thereby visualizing and developing the electrostatic latent image.
After development, the photoconductor 3b to which the toner adheres is moved to a transfer region by the rotation of the photoreceptor drum 3. In the transfer region, a transfer device 51 including a high-voltage power source 52 and a charger-type or contact-roller-type transfer charger 53 is provided, and by a paper feeder (not shown), a transfer material P such as paper is transported between the transfer device 51 and the photoconductor 3b in synchronism with a transfer timing. By the transfer device 51, a voltage of a polarity which attracts the toner on the photoconductor 3b is applied to the transfer material P transported, thereby moving the toner onto the transfer material P and transferring a toner image.
Immediately after the transfer region, a removing device 61 including: a high-voltage power source 62 and a charger-type or contact-roller-type removing charger 63 is provided, by which a voltage of the opposite polarity to the polarity at the time of transfer is applied to the transfer material P sticking to the photoreceptor drum 3 by the charge supplied at the time of transfer. After the transfer material P is removed by the removing device 61 from the photoreceptor drum 3, the toner image on the transfer material P is fixed by a fixing device 71, for example, by thermal fusion. Finished with fixing, the transfer material P is discharged out of the image forming device. In addition, the surface of the photoconductor 3b after transfer is cleaned by a cleaning device 81, and residual charges on the surface are eliminated by a static eliminator 91 such as an optical or contact static eliminator so as to electrically initialize the surface.
With regard to the foregoing image forming processes, the contact-type charging roller is frequently used for the main charging device 21 for charging the photoconductor 3b. The charging roller has a structure in which its center is a metallic core, and surrounding the metallic core is a conductive elastic roller. In order to charge the surface of the photoconductor 3b, the bias voltage is applied between the core of the charging roller and the conductive body 3a of the photoreceptor drum 3, and the elastic roller is rotated while keeping contact with the surface of the photoreceptor drum 3. A method of applying only a direct current voltage (hereinafter referred to as dc voltage) as the bias voltage is prone to troubles such as non-uniformity of charging and dirt, and also has a bad influence on the environment. Therefore, a method of superimposing an alternating current voltage (hereinafter referred to as ac voltage) on the dc voltage is pursued.
There is a problem, however, in the foregoing method of applying the bias voltage of a superimposed ac voltage, i.e. as a frequency produced by the ac voltage increases, oscillation of surrounding components such as the photoreceptor drum 3 produces a noise. When the ac voltage is applied to the charging roller, attraction due to electrostatic force between the photoreceptor drum 3 and the charging roller changes over time and acts to generate oscillation. More specifically, as the ac voltage approaches a peak (either the maximum peak or the minimum peak), attraction force (electrostatic force) increases and the elastic roller is drawn to the photoreceptor drum by undergoing elastic deformation, whereas the attraction force decreases as the ac voltage approaches the median of the peaks, which causes the elastic roller to separate from the photoreceptor drum 3 by the restoring force of the elastic roller which has undergone elastic deformation. As a result, an oscillation phenomenon is observed between the photoreceptor drum 3 and the charging roller, thereby producing a charging noise.
For the purpose of preventing the oscillation, Japanese Unexamined Patent Publication No. 142922/1993 (Tokukaihei 5-142922 published on Jun. 11, 1993) discloses a method of increasing a characteristic frequency of the photoreceptor drum by providing a coil spring as a damper in the photoreceptor drum; and Japanese Unexamined Patent Publication No. 142923/1993 (Tokukaihei 5-142923 published on Jun. 11, 1993) discloses a method of increasing the characteristic frequency of the photoreceptor drum by inserting an elastic member in the photoreceptor drum. However, it is required in these two methods to determine by experiment every condition of the member provided in the photoreceptor drum in order to suppress oscillation and thus the methods are inefficient and also non-universal in respect of conditions of suppressing oscillation. Moreover, further troubles may arise over (i) increased number of members and longer assembling time due to insertion of the members into the photoreceptor drum, (ii) poor handling of the photoreceptor drum due to increased weight, and (iii) defective damping effect due to improper mechanical fixing when the members are engaged.